Thanks
alot Mr.Max Katz for your reply ,
I have got the point , Now I am facing another problem , My form now contains only picklist which contains 400 users and only a save button , the problem is arising when the user selects all the 400 users and click on save button , saving the form takes long time around 20 minutes ???
note that there will be no problem if the number of the selected users are around 100.